It ain't the boiler, it's the piping. You can't just bug a radiant run onto existing piping or manifolds and expect to get good results. As you have seen, you sometimes get no results at all. That's because there is friction involved in that hundred foot or so of pex under there and water being fairly stupid, will always flow to the path of least resistance which unfortunately in your case, is not your radiant loop. Besides that bit of aggravation, there is also the issue of delta T across the loop to consider and that usually requires some sort of 3way zone valve, a separate circulator and primary secondary piping or a hydronic separator which is what I have been going to lately along with VFD pumping which takes the guess work out of things. So, the good news is that you have the piping installed already. The bad news is that you are going to need to do a total re-pipe of the boiler and existing zones as well as make some decisions as to control strategy because a normal thermostat will overheat the hell out of that zone also. A floor temp sensor is a better idea.